ADHD and sugar: new avenues for an old question?
There is something about sugar consumption and hyperactivity that seems very intuitive. Perhaps, because everyone has experienced kids going crazy at a birthday party after having consumed industrial amounts of sugar through cakes, sweets, and lemonade. According to questionnaire studies, this opinion is also shared by parents of children with attention deficit hyperactivity disorder (ADHD). For some parents, dietry restrictions that cut-out all refined carbohydrates are preferred over gold standard stimulant treatment (Sciutto, 2015). However, the evidence that sugar has any effect on children’s behaviour and cognitive performance is not well supported. A number of studies were conducted in the 1980s and 1990s that investigated the link between sugar consumption and behaviour in children. The general design of these studies involved having children consume food items that contained either sugar or an artificial sweetener (aspartame or saccharine) without the child or the experimenter being aware which group the individual child was assigned to (double blind experiment). Next, the outcome of the food intake was measured as performance on a particular cognitive task (mostly standard assessments of general cognitive ability). A meta-analysis of these studies found no effect on test performance or observed behaviour (Wolraich, Wilson, & White, 1995). These results suggest that there is no effect of sugar consumption on general measures of behaviour and performance in typical children. But what about children with an ADHD diagnosis?
Some studies that looked at associations between environmental influences and ADHD in a large number of people found links between a diet high in carbohydrates and saturated fats and ADHD scores in independent samples in the UK, Western Australia, and Korea (Howard et al., 2011; Woo et al., 2014). However, it is difficult to draw firm conclusions from these population-based studies as both dietary patterns and ADHD behaviours may be influenced by a common underlying factor, e.g. differences in family lifestyle or socio-economic status. Conversely, some studies investigated the effect of specific diets as a treatment for ADHD. A recent review reported no effects of sugar vs apartame/saccharine on ADHD symptoms in 3 out of 4 studies (Heilskov Rytter et al., 2015). Interestingly, the one study that found differences reported differences on a measure of attention, while a behavioural assessment of aggressive behaviour was not influenced by sugar ingestion (Wender & Solanto, 1991). The other studies were based on playroom observations or assessments of general learning and memory (Gross, 1984; Milich & Pelham, 1986; Wolraich, Milich, Stumbo, & Schultz, 1985). The possibility that sugar consumption only affects performance on certain cognitive tasks but not general behaviour remains to be further investigated. In addition, research in the last decades has established that children diagnosed with ADHD present a very heterogeneous sample. This lead to the inclusion of subtypes in the last revision Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). It is possible that different subtypes of the disorder are differently affected by sugar consumption and that different studies find different results due to differences in the inclusion of subtypes within the study sample. The scientific interest in links between sugar and ADHD has ebbed away in the new millennium, while parents and educators continue to belief in a link. It seems that future work should address the questions that have not been satisfyingly answered in the previous work.
Some support for an association between ADHD and sugar comes from studies on the physiological level. Children are typically less able to regulate blood sugar levels compared to adults and this seems to be especially the case for children with ADHD (Lindblad, Eickhoff, Forslund, Isaksson, & Gustafsson, 2015). The brain is the most energy-hungry organ consuming 25% of the energy while only accounting for 2% of the body’s mass. Differences in energy metabolism are also known to influence behaviour, e.g. in the case of hypoglycemia (Millichap & Yee, 2012). Sugar intake was also found to increased EEG beta activity over frontal regions in children with “food-induced” hyperactivity (Uhlig, Merkenschlager, Brandmaier, & Egger, 1997), which related to measures of problematic behaviour. Based on these findings and others, a recent extensive review suggested that differences in energy metabolism are at the heart of ADHD symptomatology (Killeen, Russell, & Sergeant, 2013). According to the neuroenergetics theory, ADHD is characterised by a less efficient regulation of energy supply to neurons, which results in a reduction of around 15% in energy capacity. Further, based on this model inconsistencies between studies are also expected when the demands of the task vary. However, it is currently not clear from the model how differences in neuronal energy metabolism relate to dietary patterns. It is possible that attraction to high energy foods is an attempt of the system to counter-balance inefficient energy supply, but further empirical and theoretical work will be needed to fully understand the link between dietary preferences and brain metabolism.
In summary, the discussed studies do not support that there is a strong link between sugar consumption and ADHD. However, evidence from populations studies and physiological investigations indicate that energy regulation is affected in children with ADHD. These studies highlight that ADHD needs to be understood as a complex systemic disorder that affects many different levels of observation from cell biology to behaviour, which will hopefully be addressed in future research.
This line of investigation also poses questions that are extremely important for parents and educators, but it also raises concerns about current research practices. If food intake has an immediate effect on behavioural performance that differs systematically between ADHD and comparison groups, any study that aims to investigate neuro-cognitive differences should be controlling this factor.
Photo credit: Moyan Brenn (https://www.flickr.com/photos/aigle_dore/)
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